Cities can be seen as living organism that require a vast range of inputs in order to live and work. In the past times, medieval fortified cities had the capacity to sustain themselves for quite long periods during sieges, relying only on intra moenia production and on the food and the good resources stored by the city. Nowadays, enormous amounts of resources are needed by urban settlements, and sieges or isolation are no longer a threat. Therefore, the land surface on which the city is built on is not capable to provide enough resources for all the inhabitants. Actually, its not usually able to produce something any more.
For such reason cities now require several kind of inputs, that can be broadly classified in freights (construction materials, food, etc.), water and energy. As all living organisms, the city processes its inputs and delivers some results (services, other freights, etc.) while emitting wastes in the form of solid garbage, sewage and heat.

Cities can be seen as living organism that require a vast range of inputs in order to live and work. In the past times, medieval fortified cities had the capacity to sustain themselves for quite long periods during sieges, relying only on intra moenia production and on the food and the good resources stored by the city. Nowadays, enormous amounts of resources are needed by urban settlements, and sieges or isolation are no longer a threat. Therefore, the land surface on which the city is built on is not capable to provide enough resources for all the inhabitants. Actually, its not usually able to produce something any more.
For such reason cities now require several kind of inputs, that can be broadly classified in freights (construction materials, food, etc.), water and energy. As all living organisms, the city processes its inputs and delivers some results (services, other freights, etc.) while emitting wastes in the form of solid garbage, sewage and heat.